The term “carbohydrate” or “saccharide” embraces a wide variety of chemical compounds having the general formula (CH2O)n, such as monosaccharides, disaccharides, oligosaccharides and polysaccharides. Oligosaccharides and polysaccharides are chains composed of monosaccharide units, which are also generally referred to as sugars. The monosaccharide units of an oligo- or polysaccharide can be arranged in various orders. The linkage between any two saccharide units can occur in any of approximately ten different ways. As a result, the number of different possible stereoisomeric oligosaccharide or polysaccharide chains is enormous. Saccharides are a key component of glycosphingolipids found in cell membranes. Of interest with respect to the present invention is a certain class of glycoshingolipids known as gangliosides.
Gangliosides are glycosphingolipids, often found in cell membranes, that consist of three elements. One or more sialic acid residues are attached to an oligosaccharide or carbohydrate core moiety, which in turn is attached to a hydrophobic lipid (ceramide) structure which generally is embedded in the cell membrane. The ceramide moiety includes a long chain base (LCB) portion and a fatty acid (FA) portion. Gangliosides, as well as other glycolipids and their structures in general, are discussed in, for example, Lehninger, Biochemistry (Worth Publishers, 1981) pp. 287-295 and Devlin, Textbook of Biochemistry (Wiley-Liss, 1992). Gangliosides are classified according to the number of monosaccharides in the carbohydrate moiety, as well as the number and location of sialic acid groups present in the carbohydrate moiety. Mono sialogangliosides are given the designation “GM”, disialogangliosides are designated “GD”, trisialogangliosides “GT”, and tetrasialogangliosides are designated “GQ”. Gangliosides can be classified further depending on the position or positions of the sialic acid residue or residues bound. Further classification is based on the number of saccharides present in the oligosaccharide core, with the subscript “1” designating a ganglioside that has four saccharide residues (Gal-GalNAc-Gal-Glc-Ceramide), and the subscripts “2”, “3” and “4” representing trisaccharide (GalNAc-Gal-Glc-Ceramide), disaccharide (Gal-Glc-Ceramide) and monosaccharide (Gal-Ceramide) gangliosides, respectively.
Numerous types of gangliosides found in nature have been isolated and identified and vary primarily in the basic saccharide structure (e.g. GM3, GM2, GM1, GD1a, GD1b and GT1). A variety of procedures are available for the isolation and purification of such “natural” gangliosides from organs and tissues, particularly from animal brain (Sonnino et al., 1992, J. Lipid Res., 33:1221-1226; Sonnino et al., 1988, Ind. J. Biochem. Biophys., 25:144-149; Svennerholm, 1980, Adv. Exp. Med. Biol., 125:533-44) as well as bovine buttermilk (Ren et al., 1992, J. Bio. Chem., 267:12632-12638; Takamizawa et al., 1986, J. Bio. Chem., 261:5625-5630).
Gangliosides are normal components of plasma membranes and are particularly abundant in the nervous system. In humans, gangliosides are most abundant in the gray matter of the brain, particularly in nerve endings. They are believed to be present at receptor sites for neurotransmitters, including acetylcholine, and can also act as specific receptors for other biological macromolecules, including interferon, hormones, viruses, bacterial toxins, and the like.
Certain gangliosides are found on the surface of human hematopoictic cells (Hildebrand et al. (1972) Biochim. Biophys. Acta 260: 272-278; Macher et al. (1981) J. Biol. Chem. 256: 1968-1974; Dacremont et al. Biochim. Biophys. Acta424: 315-322; Klock et al. (1981) Blood Cells 7:247) which may play a role in the terminal granulocytic differentiation of these cells. Nojiri et al. (1988) J Biol. Chem. 263: 7443-7446. These gangliosides, referred to as the “neolacto” series, have neutral core oligosaccharide structures having the formula [Galβ-(1,4)GlcNAcβ(1,3)]nGalβ(1,4)Glc, where n=1-4. Included among these neolacto series gangliosides are 3′-nLM1 (NeuAcα(2,3)Galβ(1,4)GlcNAcβ(1,3)Galβ(1,4)-Glcβ(1,1)-Ceramide) and 6′-nLM1 (NeuAcα(2,6)Galβ(1,4)GlcNAcβ(1,3)Galβ(1,4)-Glc,(1,1)-Ceramide).
It has been widely demonstrated that gangliosides are able to enhance functional recovery both in the lesioned peripheral nervous system (PNS) and the central nervous system (CNS), through the involvement of specific membrane mechanisms and the interaction with trophic factors, as pointed out from studies in vitro on neuronal cultures (Ferrari, F. et al., Dev. Brain Res., 1983, 8:215-221; Doherty, P. et al., J. Neurochem., 1985, 44:1259-1265; Skaper, S. D. et al., Mol. Neurobiol., 1989, 3:173-199). Gangliosides have been used for treatment of nervous system disorders, including cerebral ischemic strokes. See, e.g., Mahadnik et al. (1988) Drug Development Res. 15: 337-360; U.S. Pat. Nos. 4,710,490 and 4,347,244; Horowitz (1988) Adv. Exp. Med. and Biol. 174: 593-600; Karpiatz et al. (1984) Adv. Exp. Med. and Biol. 174: 489-497.
As a result, attempts have been made to use gangliosides in the treatment of disorders of the nervous system. This has led to the development of synthetic gangliosides as well as natural ganglioside containing compositions for use in the treatment of disorders of the nervous system (U.S. Pat. Nos. 4,476,119, 4,593,091, 4,639,437, 4,707,469, 4,713,374, 4,716,223, 4,849,413, 4,940,694, 5,045,532, 5,135,921, 5,183,807, 5,190,925, 5,210,185, 5,218,094, 5,229,373, 5,260,464, 5,264,424, 5,350,841, 5,424,294, 5,484,775, 5,519,007, 5,521,164, 5,523,294, 5,677,285, 5,792,858, 5,795,869, and 5,849,717).
Gangliosides have also been implicated as playing a significant role in certain types of cancer. Neuroblastoma is a form of cancer that primarily afflicts children under the age of five. Individuals suffering from neuroblastoma may have tumors growing near the spinal cord, and very large tumors have been found to cause paralysis in such patients. Gangliosides have been shown to play a role in both the growth and the inhibition of the growth of neuroblastoma-associated tumors (Basavarajappa et al., 1997, Alcohol Clin. Exp. Res., 21(7):1199-203; Singleton et al., 2000, Int. J. Dev. Neurosci., 2000, 18(8):797-80).
However, there still exists a need in the art for compounds capable of acting as neuroprotective agents in a manner similar to or better than the natural gangliosides for the prophylaxis, treatment and cure of disorders of the nervous system. Further, differences in the structure of ganglioside compounds can refine the structure-function relationship of such compounds to provide powerful tools for control of the growth of certain kinds of tumors, including neuroblastoma tumors.